Method and apparatus for producing a welded joint in thermoplastic strap with differential pressure

ABSTRACT

A novel method and apparatus are provided for making an improved welded joint between overlapping lengths of thermoplastic strap. The welded joint includes a central region and an adjacent peripheral region. The pressure on the strap joint during formation is released first in the peripheral region and subsequently in the central region. An embodiment of tile apparatus for making such a joint includes an anvil, a central strap-engaging member, a peripheral strap engaging member, and a mechanism for moving the anvil subsequently to three positions. At a first position the overlapping strap lengths are pressed between the anvil and both the central and peripheral strap-engaging members. In the second position of the anvil the peripheral strap-engaging member does not press against the strap lengths while the central strap-engaging member continues to press the strap lengths against the anvil. In the third position of the anvil, the anvil and strap-engaging members are sufficiently spaced apart to release all of the pressure on the strap lengths.

This application is a division of application Ser. No. 969,584, filedOct. 30, 1992 now abandoned.

TECHNICAL FIELD

This invention relates to welds for securing together overlappingportions of thermoplastic strapping and also relates to a method andapparatus for producing such welds.

BACKGROUND OF THE INVENTION AND TECHNICAL PROBLEMS POSED BY THE PRIORART

A variety of processes have been marketed and/or proposed over the yearsfor welding or bonding together overlapping portions of a tensioned loopof thermoplastic strap encircling an article. In the context of thissubject matter, the terms "joint," "weld," and "welded joint" areconventionally used interchangeably to designate the bonded orjoined-together portions of the strap.

In one such process, a hot blade is employed to melt the interfaceportions of the overlapping strap lengths which then resolidify to formthe weld. However, this process typically generates vapor and smokewhich may be objectionable. Thus, it would be desirable to provide animproved process wherein smoke generation is eliminated or substantiallyreduced.

Another process is effected by first pressing the overlapping strapportions together and then creating a unidirectional or multidirectionalbodily sliding frictional movement between the contacting surfaces ofthe overlapping strap portions so as to melt the interface region of theoverlapping strap portions. The melted interface region is allowed tosolidify at rest, either under pressure or free of pressure, so as tobond the overlapping strap portions together.

This process, which can be generally designated as "friction-fusionwelding" or "friction welding", has proven to be especially effectivewith conventional thermoplastic strap materials such as nylon,polyester, and polypropylene. Such conventional straps are typicallyprovided commercially in widths ranging from about 5 mm. to about 13 mm.and in thicknesses ranging between about 0.25 mm. and about 0.89 mm.Some systems for making a friction-fusion weld between portions of suchstrap employ high-speed, reciprocating mechanisms, and these can produceconsiderable noise during operation. It would be desirable to provide animproved system which would produce an effective friction-fusion weldwith substantially less noise.

Welded joints on thermoplastic straps have found wide commercialacceptance in many applications. However, a welded joint is typicallythe weakest part of a tensioned strap loop secured about a package orother object. Attempts have been made to produce stronger welds.

Unique strap joints or welds having greater strength, and methods forproducing such welds, are disclosed in U.S. Pat. Nos. 4,707,390 and4,776,905. The weld is formed by first fusing (melting or liquefyingunder the application of heat) at least part of the thickness of eachstrap portion across the width of the interface. The fused parts of thestrap portions are then permitted to subsequently solidify to form thewelded joint. The weld is created with a selected strap material in amanner such that cavities (such as voids, bubbles, or discrete volumesof a separate material) are encapsulated within the resolidified regionof the weld. The cavities have been found to result in the weld having agreater strength as discussed hereinafter.

The cavities are dispersed generally across the width of the strap weldand are preferably more generally concentrated toward each longitudinalend of the weld. In the preferred form of the weld, the concentration ofcavities in the middle portion of the weld is substantially less than atthe ends of the weld.

In one form of the method for producing the improved joint of theinvention, the cavities are believed to result from the production ofgaseous bubbles during the welding process. It is believed that sometypes of straps contain significant amounts of an additional material,such as moisture, which can exist as a gas when the strap portions aremelted under pressure. For example, polyester and polyamide nylon strapsare hygroscopic and can contain some amount of water.

It is theorized in U.S. Pat. No. 4,707,390 that when portions of suchstraps are melted and are under pressure during welding, the generatedgas bubbles tend to be forced outwardly toward the edges and ends of theweld. If the welding pressure is terminated while the strap portions arestill molten and before all of the bubbles have been squeezed out of theweld area, the remaining bubbles become encapsulated within thesolidifying strap material to form the cavities.

The cavities may be defined by other structures and materials which areprovided as part of the strap structure as disclosed in the U.S. Pat.No. 4,892,768. In particular, the strap can include (1) at least a firstlayer of a first material that does not produce the desired cavities,and (2) a second material for effecting the creation of the plurality ofdiscreet volumes in the resolidified region as a result of theapplication of heat and subsequent resolidification.

In one embodiment, the second material is present on at least one sidesurface of the first material layer, and the second material defines asecond layer carried on the first material layer. An example of thefirst material is polypropylene or polyethylene terephthalate. Anexample of the second material is polyethylene terephthalate having anintrinsic viscosity of about 1.0.

In another embodiment, the second material is dispersed as an additivein the first material layer and has a greater concentration at least atone side surface of the first material layer. The additive secondmaterial may be a foaming agent material such as a 5-phenyltetrazolecompound or a toluenesulfonyl semicarbicide compound.

In another embodiment, the second material may disperse to form separateglobules which occupy the discreet volumes. Such a second material maybe the combination of polyisobutylene dispersed in a layer ofpolypropylene which in turn is carried on the first material layer.

With appropriate welding techniques, the above-described strapcompositions can produce welded joints with cavities distributedthroughout the weld, including the outer or end portions of the weld.The above-referenced U.S. Pat. Nos. 4,707,390 and 4,892,768 eachdisclose that the exact mechanism by which the cavities increase theweld strength is not necessarily fully or accurately understood, butnevertheless propose a theory. In particular, it is believed that theimproved joint strength of the weld results from the redistribution ofstresses within the weld, and that the cavities cause the stressredistribution. It is believed that welds tend to fail when cracks format the ends of the weld. The cavities are believed to reduce thestresses at a crack tip that is propagating into one or more of thecavities.

While the above-described type of cavity-containing weld providesincreased strength and functions well in a variety of applications,there is a continuing need for a system that can produce high qualitywelds routinely and consistently, and in an economic manner.

In particular, it has been found in some applications that, depending onthe strap material, strap size, and welding parameters, the capabilityfor repeatedly producing acceptable welds having desired high strengthis not as good as would be desired. It has been theorized that in somesuch cases, the cavities may not be distributed in a manner thatprovides the desired level of increased joint strength. For example,there may be too many cavities in the central portion of the weld. Thecentral portion of the weld may then have an excessively "foamed"structure while the peripheral region of the weld has too few cavities.It is suspected that, in some situations, this might actually decreasethe strength of the weld. Accordingly, it would be desirable to providea system for better controlling the creation and distribution of thecavities the weld.

The present invention provides an improved welded joint betweenoverlapping strap lengths and also provides an apparatus and methodwhich can accommodate designs having the above-discussed benefits andfeatures.

SUMMARY OF THE INVENTION

Although the novel method and apparatus of the present invention may beadvantageously employed with certain types of straps to produce a weldedjoint containing encapsulated cavities, it will be appreciated that thenovel method and apparatus may be employed with other types of strap forforming welded joints which do not contain encapsulated cavities.

According to one aspect of the present invention, an improved weldedjoint is provided between two overlapping lengths of thermoplastic strapwhich are arranged in a face-to-face relationship at the welded joint.At least part of the thickness of each strap length is resolidified froma fused state which has been created by means of above-ambient pressure.The resolidified thickness part of one of the strap lengths is mergedwith the resolidified thickness part of the other strap length to definea continuous resolidified region. The resolidified region includes aperipheral region and an adjacent central region in which theabove-ambient pressure was maintained during the fused state longer thanin the peripheral region.

The welded joint may also be characterized as incorporating aresolidified thickness of part of one strap length being merged with aresolidified thickness of the other strap length to define a continuousresolidified region wherein the overlapping lengths of the straps in thefused state were subjected to above-ambient pressure in a central regionand in an adjacent peripheral region, but with the above-ambientpressure being relieved firstly in the peripheral region and thensubsequently in the central region.

A further aspect of the invention includes a method for making thewelded joint. The method includes the step of forcing the strap lengthsinto face-to-face contact at above-ambient pressure along an interfacein a central region and in an adjacent peripheral region.

The temperature of the strap lengths is increased along the interfacesufficiently to melt at least part of the thickness of each strapportion at the interface.

The above-ambient pressure is released firstly in the peripheral regionand subsequently in the central region whereby the melted parts of thestrap lengths along the interface solidify to form the welded joint.

According to a still further aspect of the invention, a novel apparatusis provided for making the welded joint between the first and secondoverlapping lengths of flexible, thermoplastic strap. The apparatus maybe generally characterized as including three systems. First, theapparatus includes pressing means for forcing the strap lengths intoface-to-face contact at above-ambient pressure along an interface in acentral region and in an adjacent peripheral region. Second, theapparatus includes means for increasing the temperature of the straplengths along the interface sufficiently to melt at least part of thethickness of each strap portion at the interface. Finally, there is ameans for releasing the above-ambient pressure firstly in the peripheralregion and subsequently in the central region. The apparatus permits themelted parts of the strap lengths along the interface to cool so as tosolidify and form the welded joint.

A preferred embodiment of the apparatus includes an anvil for engagingthe first overlapping length of strap. The apparatus further includes acentral strap-engaging member for engaging the second overlapping lengthof strap. A peripheral strap-engaging member is provided for engagingthe second overlapping length of strap around the central strap-engagingmember.

The apparatus employs pressing means for effecting relative movementbetween the anvil on the one hand and each of the strap-engaging memberson the other hand so as to establish a number of positions:

(1) a first position in which the anvil engages the first overlappinglength of strap while both the central and peripheral strap-engagingmembers engage the second overlapping length of strap to force thelengths of strap into face-to-face contact under pressure,

(2) a second position in which the anvil and the peripheralstrap-engaging member are sufficiently spaced apart to release thepressure of the peripheral strap-engaging member on the lengths of strapwhile still maintaining the pressure of the central strap engagingmember on the lengths of strap, and

(3) a third position in which the anvil and the central strap-engagingmember are sufficiently spaced apart to release the pressure of thecentral strap-engaging member on the lengths of strap.

Finally, the apparatus includes means for increasing the temperature ofthe contacting faces of the strap lengths to melt at least a part of thethickness of each strap length.

It has been found that the above-described apparatus and method forforming the welded joint can produce a welded joint as hereinbeforedescribed. With certain strap materials, the peripheral region of thewelded joint contains a distribution of cavities while the centralregion of the joint is substantially free of such cavities. Such a jointstructure has been found to have increased strength and increasedresistance to crack propagation.

BRIEF DESCRIPTION OF THE DRAWINGS

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention, from the claims, and from the accompanying drawings, in whichlike reference characters designate like or corresponding partsthroughout the several views, and wherein:

FIG. 1 is an enlarged, fragmentary, perspective view, of a welded jointbetween two overlapping lengths of transparent, thermoplastic straparranged in a face-to-face relationship, and FIG. 1 shows features ofthe welded joint in an exaggerated size and relationship so that certaindetails may be more clearly viewed;

FIG. 2 is a simplified, substantially diagrammatic, perspective view ofone form of an apparatus which is capable of producing an improvedthermoplastic strap welded joint, such as is illustrated in FIG. 1, theapparatus being shown in an initial rest mode for accommodating thepositioning of a strap loop around an article (shown in phantom bydashed lines) so that the overlapping lengths of strap can besubsequently joined together with a friction-fusion welded joint;

FIG. 3 is a fragmentary, greatly enlarged, plan view of the apparatus;

FIG. 4 is a fragmentary, cross-sectional view taken generally along theplane 4--4 in FIG. 3;

FIG. 5 is a simplified, exploded, perspective view of the internalcomponents of the apparatus; and

FIGS. 6, 8, and 10 are each views similar to FIG. 3 but showing thesequence of operation of the apparatus, and FIGS. 7, 9, and 11 are eachviews similar to FIG. 4 and correspond to the operational sequence shownin FIGS. 6, 8, and 10, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While this invention is susceptible of embodiment in many differentforms, this specification and the accompanying drawings disclose onlyone specific welded joint, one basic method for making the joint, and apreferred embodiment of the apparatus for effecting the method. Theseare presented as examples of the invention. The invention is notintended to be limited to the examples so described, however. The scopeof the invention is determined by the appended claims.

For ease of description, the apparatus of this invention is described ina normal (upright) operating position, and terms such as upper, lower,horizontal, and the like, are used with reference to this position. Itwill be understood, however, that the apparatus of this invention may bemanufactured, stored, transported, used, and sold in an orientationother than the position described.

Some of the figures illustrating an embodiment of the apparatus showstructural details and mechanical elements that will be recognized byone skilled in the art. However, the detailed descriptions of suchelements are not necessary to an understanding of the invention, andaccordingly, are not herein presented.

The apparatus of the present invention may be characterized as anassembly for incorporation in a tool or machine for effecting afriction-fusion weld of overlapping lengths of thermoplastic strap. Suchstrapping machines or tools can employ certain conventional componentsthe details of which, although not fully illustrated or described, willbe apparent to those having skill in the art and an understanding of thenecessary functions of such components.

An embodiment of the welded joint of the present invention isillustrated generally in FIG. 1. The joint is formed in overlappingportions of a strap S. Typically, the strap S may be provided in theform of a length of strap for being disposed in a loop around an articleA (FIG. 2). The strap S has the flexibility, strength, abrasionresistance, and compositional stability, among other characteristics,that would be suitable for use as strap in the particular applicationfor which it is intended.

Conventional thermoplastic straps include nylon, polypropylene, andpolyethylene terephthalate. Straps formed from such materials may befabricated by generally similar techniques, as is disclosed, forexample, in U.S. Pat. No. 3,394,045 relating to polypropylene strapping.

Conventional strapping materials are commercially available from variousentities around the world, including sources in the United States ofAmerica. For example, polyethylene terephthalate strap is sold in theUnited States of America under the product designation "Tenax B" strapby Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025,U.S.A. This is one type of strap that may be employed where it isdesired to produce an improved welded joint having encapsulated cavitieswhich increase the joint strength. "Tenax B" strap has an intrinsicviscosity of about 0.95. Improved welds have also been produced inpolyethylene terephthalate strap which has an intrinsic viscosity ofabout 0.62 and which is sold by Signode Corporation under the productdesignation "Tenax A" strap. These materials have the capability ofbeing provided with varying degrees of uniaxial orientation sufficientto provide the strengths desired for a variety of specific conventionalstrapping applications.

The welded joint shown in FIG. 1 has been illustrated as being formed inthe strap S of the Tenax B brand material described above. The strap Sincludes a first, or upper, strap length U and a second, or lower, straplength L. The strap lengths U and L are positioned in an overlappingrelationship along or in an elongate strap-receiving path. A part of thethickness of each strap portion U and L forms a resolidified region 14which constitutes the weld. The part of the thickness of each strapportion that forms the weld is resolidified from a fused state. Theresolidified thickness part of the upper strap length U is merged withthe resolidified thickness part of the lower strap length L.

The weld includes a central region C which, in the embodimentillustrated in FIG. 1 has a generally circular configuration. Aperipheral region P is located beyond the central region C. In thepreferred embodiment of the welded joint illustrated in FIG. 1, theperipheral region P encircles the central region C. The peripheralregion P has a generally annular configuration except that the portionsof the peripheral region P are "cut off" at the lateral side edges ofthe strap S. However, if a wider strap S was employed, a peripheralregion P having a generally full, annular configuration would beaccommodated.

The peripheral region P contains a plurality of cavities 16 (such asvoids, bubbles, or discreet volumes of a separate material) which areencapsulated within the peripheral portion of the resolidified region14. On the other hand, the central region C is substantially free ofsuch cavities 16. As will be explained hereinafter, a novel method forproducing the welded joint illustrated in FIG. 1 results insubstantially all of the cavities 16 being distributed in the peripheralregion P with substantially no cavities 16 existing in the centralregion C.

As explained above, it is believed that the improved joint strength ofthe weld results from the redistribution stresses within the weld, andthat the cavities cause the stress redistribution. It is believed thatthe cavities reduce the stresses at a crack tip that may beginpropagating into one or more of the cavities. The exact mechanism bywhich the cavities 16 function to increase the joint strength is notnecessarily fully understood, and there is no intent herein to be boundby any theory or by any explanation as provided above. The number, size,and distribution of the cavities 16 within the peripheral region P in aparticular weld can vary and may depend, to some extent, on the widthand thickness of the strap and weld, as well as upon the particularcomposition of the strap S and specific weld techniques employed.

The nominal thickness of each strap length U and L may be, for example,about 0.5 mm., and the width of each strap length U and L may be about11 mm. The length of the completed weld, along the length of the strapS, may be about 17 mm. The thickness of the welded joint, that is, thethickness of the resolidified region 14, would typically be about 0.1mm. for this type and size of strap S. These dimensions would, ofcourse, vary depending upon the size of the strap employed and upon themethod and apparatus employed for creating the welded joint.

For the typical strap S and weld illustrated in FIG. 1, the cavities 16generally each have a volumetric configuration that is irregular and notperfectly spherical. The cavities 16 typically do not all have the samesize. For example, a substantial number of the cavities may have a majordimension falling in the range of about 25% to about 50% of thethickness of the resolidified region 14. However, the size distributionof the cavities 16 is, of course, dependent upon the strap material,strap dimensions, and specific method employed for creating the weldedjoint.

When the welded joint is produced with the preferred embodiment of theapparatus described hereinafter, the upper strap length U is engaged bythe apparatus in a way that creates a pattern of gripping marks orindentations 20 in the upper surface of the strap length U. Theseindentations 20 are not part of the welded joint per se, and thepresence or absence of such indentations, or the shape and arrangementof such indentations, is dependent upon the manner in which the weldedjoint is made.

The welded joint illustrated in FIG. 1 may be made by the apparatus 30illustrated in FIGS. 2-11. The apparatus 30 is shown in FIG. 2 in anorientation that the apparatus would have in a strapping machine onwhich, or in which, an article A would be positioned. For ease ofillustration, the other components of a conventional strapping machinehave been omitted.

The apparatus 30 includes an anvil 32 for engaging the first overlappinglength of strap U. The anvil 32, in the preferred embodimentillustrated, is located at the top of the apparatus 30 and is maintainedthere by the frame of the machine (not illustrated). Preferably, thelower surface of the anvil 32 has serrations or teeth 34 (FIG. 4) forfirmly engaging the upper strap length U. The top surface of the anvilmay be smooth.

Typically, the article A is placed on the top of the anvil 32 andsurrounding portions of the machine so that the strap S can be loopedaround the article A as illustrated in FIG. 2. A portion of the upperstrap length U can be gripped (by conventional means not illustrated) sothat the machine can pull the lower strap length L to tension the straploop tightly about the article A. The tensioning means may include anysuitable special or conventional mechanism (not illustrated), and such amechanism forms no part of the present invention. After the loop hasbeen tensioned, the overlapping strap lengths U and L are joinedtogether by a friction-fusion weld produced by the apparatus 30 asexplained in detail hereinafter.

When the strap S is initially encircled about the article A, theoverlapping strap lengths U and L may be initially relatively looselydisposed beneath the anvil 32 as shown in FIG. 4, but the upper straplength U and lower strap length L may be suitably gripped by special orconventional means on the machine (not illustrated) to maintain thestrap loop under tension on either side of the anvil 32.

In one conventional system typically employed in strapping machines, thestrap S is drawn from a supply reel (not illustrated) so that initiallya trailing portion E of the lower strap length L extends from the regionof strap overlap through a tensioning mechanism (not illustrated) and tothe supply reel. The tensioning mechanism may be operated to maintainthe loop tension in conjunction with a suitable gripping system forholding the upper strap length U in place.

The anvil 32 is movable vertically by suitable positioning means orpressing means schematically illustrated by the double headed arrow 33in FIG. 2. Depending upon the particular step in the operation of theapparatus, the anvil 32 may be maintained at a first or lowest elevation(FIGS. 6 and 7), a second or intermediate position (FIGS. 8 and 9), anda third or highest position (FIGS. 3, 4, 10, and 11). The verticalmovement of the anvil 32 may be effected by any suitable special orconventional mechanism (not illustrated), and the details of the designand operation of such a mechanism form no part of the present invention.

The anvil 32 may also be movable in the plane parallel to the bottom ofthe article A. In particular, the anvil 32 is preferably movable(perpendicular to the plane of FIG. 4) between an extended positionillustrated in solid lines in FIGS. 2 and 3 and a retracted positionshown in phantom by dashed lines 36 in FIG. 3. In the extended position,the anvil 32 overlies the overlapping strap lengths U and L, and in theretracted position, the anvil 32 is spaced away from the overlappingstrap lengths U and L. The anvil 32 is typically retracted after thewelded joint has been made in the strap loop so that the strappedarticle A can be easily removed from the strapping machine. The anvil 32may be extended and retracted by suitable special or conventionalmechanisms (not illustrated), the details of which form no part of thepresent invention.

The apparatus 30 includes a supporting frame 40 below the anvil 32. Theframe 40 defines a cylindrical cavity 42. The frame also includes abearing 44 mounted within the cavity 42.

A drive shaft 46 is adapted to be mounted on the bearing 44 within thecavity 42 for rotation about a vertical axis 48. The drive shaft 46 hasan enlarged upper portion 50 defining a cavity 54. The cavity 54 is acylindrical drive cavity having a rotational axis 58 which is parallelto, but radially offset by a distance X from, the drive shaft axis 48.The lower end of the drive shaft 46 has an appropriate configuration(FIG. 5) for being engaged with a suitable rotating means, such as anelectric motor drive system (not illustrated). The drive shaft 46 canthus be rotated about the longitudinal axis 48.

Before the drive shaft 46 is mounted in the frame cavity 42 on thebearing 44 and connected to the drive system, various internalcomponents are assembled in the drive shaft cavity 54. Specifically, onecomponent to be disposed in the cavity 54 is a combination roller ballthrust bearing and needle bearing unit 60. The unit 60 may be a suitablespecial or conventional device that defines a central, cylindrical bore64 (FIG.4). The device 60 may also include a peripheral retaining groove62. One side of the drive shaft upper portion 50 defines a slot 68 (FIG.5) for receiving a C-shaped keeper clip 70. When the bearing unit 60 isdisposed within the drive shaft cavity 54, the keeper clip 70 can beinserted into the drive shaft slot 68 to engage a portion of the groove62 of the bearing unit 60 to retain the bearing unit 60 in position.

Before the bearing unit 60 is disposed in the drive shaft 46, a numberof other components are mounted in the bearing unit 60. In particular,as shown in FIG. 5, the bearing unit bore 64 receives a vibrator unit 76which includes a shaft 78 and an enlarged disk-shaped, peripheralstrap-engaging member 80. The shaft 78 is adapted to be received withinthe bore 64 of the bearing unit 60. A felt ring 83 is preferablyprovided for being disposed on the vibrator unit shaft 78 below theperipheral strap-engaging member 80 before the vibrator unit 76 ismounted in the bearing unit 60.

The bottom of the shaft 78 defines a groove 82 for receiving a largesnap ring 84. Before the snap ring 84 is installed, but after thevibrator unit 76 has been mounted within the bearing 60, a thrust washer86 is disposed on the shaft 78 below the bearing unit 60. The snap ring84 is then installed on the shaft 78 to retain the thrust washer 86 inplace.

The strap-engaging member 80 of vibrator unit 76 defines an annularstrap-engaging surface 112 around a central bore 90. The bore 90 has areduced diameter intermediate portion 91 (FIG. 4) and a further reduceddiameter lower portion 93 (FIG. 4) for receiving a plunger 92. Theplunger 92 includes a shaft 94 and an enlarged, central strap-engagingmember 96. A helical, compression spring 98 is disposed around theplunger shaft 94, and the plunger 92 and spring 98 are inserted into thebores 90, 91, and 93 of the vibrator unit 76 as shown in FIG. 4.

The bottom end of the plunger shaft 94 projects below the bottom end ofthe vibrator unit 76. Near the bottom end of the plunger shaft 94 thereis a groove 102 (FIG. 5) for receiving a snap ring 104. The snap ring104 bears against the bottom end of the vibrator unit shaft 78 (FIG. 4)and retains the plunger 92 within the vibrator unit 76.

After the plunger 92, spring 98, vibrator unit 76, thrust washer 86, andsnap rings 84 and 104 are properly assembled, the assembly is disposedwithin the offset bore 54 in the drive shaft 46. The keeper clip 70 isthen inserted into the drive shaft slot 68 to retain the bearing unit 60in position within the drive shaft 46.

Finally, an annular retention ring 108 is provided for being disposed onthe top portion of the drive shaft 46 to maintain the keeper clip 70 inthe slot 68. The upper edge of the ring 108 projects along side theperipheral strap-engaging member 80 but does not project beyond thestrap-engaging surface 112.

After the components are assembled in the drive shaft 46, the driveshaft 46 can be mounted within the frame 40 and connected with the drivemeans (not illustrated).

In the assembled, rest position illustrated in FIGS. 3 and 4, the spring98 biases the plunger 92 to the uppermost position as determined by theengagement of the snap ring 104 with the bottom of the vibrator unit 76(FIG. 4). The top of the plunger strap-engaging member 96 projectsupwardly above the top of the vibrator member strap-engaging member 80.The top of the plunger strap-engaging member 96 will thus engage thelower strap length L first, and will hold the strap length L off of theperipheral strap-engaging member 80 unless and until the plunger 92 isforced downwardly a sufficient amount.

In the preferred embodiment illustrated, the central strap-engagingmember 96 of the plunger 92 has a generally smooth top surface 110.However, the peripheral strap-engaging top surface 112 of the vibratorunit 76 is preferably toothed for gripping the lower overlapping strapL. In the presently contemplated preferred embodiment, the teeth aredefined on the annular surface 112 in a substantially regular array, andthe teeth are aligned in orthogonal rows. Each tooth has four exteriorsides which converge inwardly to a point, and each side is inclined atan angle of about 60°. Each tooth has a height of about 0.35 mm.Adjacent teeth in each row are spaced apart such that the tip-to-tipspacing is about 0.75 mm. and the base-to-base spacing is about 0.35 mm.The configuration and pattern of the teeth 34 on the anvil 32 may be thesame or similar. Alternatively, other suitable tooth configurations andpatterns may be employed.

The apparatus 30 can be conveniently operated in an efficient manner toproduce the friction-fusion weld at a reduced noise level and withlittle or no generation of smoke. In particular, the anvil 32 isinitially moved from the fully elevated position shown in FIGS. 3 and 4to the fully lowered position shown in FIGS. 6 and 7. As illustrated inFIG. 7, the anvil 32 is moved downwardly in the direction of the arrow126 toward the upwardly facing strap-contacting surface 110 of theplunger 92.

In the fully lowered position, the anvil 32 engages the firstoverlapping length of strap U and forces it downwardly against the lowerstrap length L so that the bottom surface of the lower strap length Lengages the plunger strap-contacting surface 110. Continued downwardmovement of the anvil 32 forces the plunger 92 downwardly, against thecompression force of the spring 98, until the strap-contacting surface110 is substantially coplaner with the peripheral strap-contactingsurface 112 of the annular strap-engaging member 80. In this manner, thestrap lengths U and L remain forced into face-to-face contact underpressure between the anvil 32 and strap-engaging surfaces 110 and 112.The anvil teeth 34 grip the top surface of the first strap U to hold itimmobile. The teeth of the peripheral strap-contacting surface 112 gripthe bottom surface of the lower strap length L so that the strap lengthL can move with the strap-contacting surface 112 and slide along, andrelative to, the immobile upper strap length U.

In the preferred form of operation, the drive shaft 46 is continuouslyrotated, and the drive shaft 46 rotates even when the anvil 32 is in thefully elevated position (FIG. 4) before the strap lengths U and L arepressed against the strap-contacting surfaces 110 and 112. Because thestrap-contacting surfaces 110 and 112 are defined on members for whichthe axis of rotation 58 is offset from the drive shaft rotational axis48, the strap-contacting surfaces are moved in an orbit about the shaftaxis 48. The orbital motion is, of course, defined by the offsetdistance X between the axis 58 and the drive shaft axis 48, and themaximum displacement is equal to twice the offset distance X.

When the strap lengths U and L are pressed together with the lower strapL engaged by the strap-contacting surfaces 110 and 112 as shown in FIG.7, the gripped lower strap-length L is subjected to orbital motion. Therelative bodily sliding movement effected between the two strap lengthsU and L by the motion of the strap length L causes at the interface amelting of a portion of the thickness of each strap length. The meltedportions of the strap lengths merge.

Next, while the drive shaft 46 continues to rotate and effectoscillation of the lower strap length L, and while the strap lengthsremain molten at the interface, the anvil 32 is moved upwardly in thedirection of the arrow 132 as illustrated in FIG. 9. The anvil 32 ismoved upwardly to an intermediate position in which the anvil 32 andperipheral strap-contacting surface 112 are sufficiently spaced apart torelease the peripheral pressure on the strap lengths. However, the anvil32 is maintained at a sufficiently low elevation so that the plunger 92is biased outwardly by the spring 98 so as to maintain engagement of thecentral strap-contacting surface 110 with the bottom surface of thelower strap length L. This maintains pressure in a central weld regionof the strap lengths.

The plunger 92 is still being moved in an orbit by the rotating driveshaft 46, but the plunger contacting surface 110, which is smooth,slides against the bottom surface of the lower strap length L. Thus, thelower strap length L, while subjected to the upwardly directed force ofthe plunger 92, remains fixed relative to the upper strap length U. Anyfrictional forces exerted by the smooth plunger 92 on the strap length Lare not sufficient to overcome the viscous engagement of the meltedportions of the strap lengths at the interface.

The force exerted by the plunger 92 on the strap lengths U and I, ismaintained for a predetermined period of time. The plunger 92 need notremain engaged until the melted strap portions completely resolidify andform the weld. However, the upward force of the plunger 92 against thestraps in the central region of the weld does provide unique advantagesunder those conditions wherein a weld is .to be formed with a pluralityof encapsulated cavities. For example, when the strap is a polyesterstrap, such as the above-described Tenax B brand strap, a plurality ofcavities or bubbles of vapor may be formed in the melted strap portions.In some situations excessive amounts of vapor may produce excessivebubbles in parts of a weld which could reduce the weld strength.However, it has been found that with the apparatus of the presentinvention, the plunger 92 subjects the central portion of the meltedstrap material to a continuing pressure, and this is believed to forcecavities or bubbles radially outwardly to peripheral portions of themelted interface material. This tends to result in a generally annulardistribution of bubbles around a central region which has few or nobubbles.

When a weld is thus produced with a plurality of such cavities, the weldconfiguration would appear substantially as diagrammatically illustratedin FIG. 1 wherein the weld central portion C contains substantially nocavities or bubbles 16 and wherein the peripheral regions P contain aconcentration of such bubbles 16. The indentations 20 in the uppersurface of the strap illustrated in FIG. 1 would typically result fromengagement of the strap upper length U with the anvil teeth 34. It isbelieved that the number of bubbles in the peripheral region compared tothe number of bubbles in the central region depends upon, among otherthings, the type of strap material, the pressures employed, and thelength of time during which the plunger 92 is maintained against thestraps while the peripheral strap-contacting surface 112 is disengagedfrom the straps.

After a predetermined time period, the anvil 32 is moved furtherupwardly, to the initial, fully raised position as illustrated in FIGS.10 and 11. The arrow 138 in FIG. 11 illustrates the direction of thefurther upward movement of the anvil 32. The pressure exerted on thecentral portion of the strap weld by the plunger 92 is thus terminated.

As the anvil 32 is moved upwardly to the fully elevated position, theoverlapping straps U and L may remain disposed against the bottomsurface of the anvil 32. This might occur if the upper strap length Ustuck to the embedded teeth of the anvil 32.

The anvil 32 may be returned to the fully elevated position (FIG. 11)before the fused portions of the overlapping strap lengths havecompletely resolidified. Alternatively, the anvil 32 may be maintainedin the intermediate position (FIG. 9) so as to maintain the centralportion of the weld region of the straps under pressure until the fusedportions of the strap material have substantially or completelyresolidified throughout the entire weld region.

During the raising of the anvil 32 to the fully elevated positionillustrated in FIG. 11, the drive shaft 46 may continue to rotate.Preferably, the drive shaft 46 rotates continually. This avoids havingto start and stop the rotating mechanisms. This can reduce the wear onthe machine and permit faster cyclic operation because there will be noneed to wait for the components to be brought up to the maximum designoscillation speed. Typically, the drive shaft rotation would beterminated only when the operator contemplates that there will be anextended period of time before the next strap portions are to be welded.

In a typical strapping machine application, where the strap is providedas a continuous length from a supply reel, the trailing portion of thestrap (portion E in FIG. 1) would be severed, by appropriate mechanisms(not illustrated). The severing of the trailing portion of the strap mayoccur prior to, or after, the formation of the welded strap joint,depending upon the type of strapping machine design employed.

In one form of the invention as presently contemplated, the apparatus 30may be operated to rotate the drive shaft at a speed of between about16,000 revolutions per minute and about 20,000 revolutions per minute.The pressure exerted by the peripheral strap-contacting surface 112 ispreferably between about 500 pounds per square inch and about 1,000pounds per square inch. The pressure exerted by the central plungerstrap-contacting surface 110 is between about 100 pounds per square inchand about 300 pounds per square inch. The peripheral contacting surface112 is engaged with the lower strap length L for a period of timeranging between about 100 milliseconds and about 1,000 milliseconds. Theplunger 92 is engaged with the lower strap length for a period of timeranging between about 50 milliseconds and about 300 milliseconds longerthan the period of engagement of the peripheral contacting surface 112.The eccentric offset distance X is between about 0.5 mm. and about 0.8mm.

Operation of the system within the above-listed parameters could beemployed to provide welds in the above-described Tenax B brand straphaving a width of about 11 mm. and a thickness of about 0.5 mm. For thisstrap, the diameter of the central plunger contact surface 110 may beabout 7.9 mm., the inner diameter of the annular contacting surface 112may be about 8 mm., and the outer diameter of the annularstrap-contacting surface 112 may be about 15.7 mm. The toothconfiguration of the strap-contacting surfaces 34 and 112 can be asdescribed above.

Of course, the above-listed operating parameters (for example, speed,pressure, and the like) are applicable to one form of the invention asan example. The parameters may be changed depending upon, among otherthings, the specific desired characteristics of the joint weld, thestrap composition, the strap size, the sizes of the strap-contactingsurfaces of the apparatus, and the like.

The preferred embodiment of the apparatus illustrated in FIGS. 2-11 ishighly effective in providing an efficient, quiet, and substantiallysmokeless process for making a welded joint between two overlappinglengths of thermoplastic strap. The process may be generallycharacterized as including a first step of forcing the strap lengths inface-to-face contact at above-ambient pressure along an interface in acentral region (region C in FIG. 1) and in an adjacent peripheral region(region P in FIG. 1). Next, the temperature of the strap lengths isincreased along the interface sufficient to melt at least part of thethickness of each strap portion at the interface. Finally, theabove-ambient pressure is released first in the peripheral region andthen subsequently in the central region. During this process, after thetemperature has been raised an amount sufficient to melt the strapthicknesses at the interface, the strap portions are allowed to cool.The melted parts of the strap lengths along the interface then solidifyto form a welded joint. The cooling may be initiated, and may even becompleted, prior to releasing the pressure in the peripheral region.

Although the above-described process is advantageously effective withthe preferred embodiment of the apparatus 30 described above withreference to FIGS. 2-11, other suitable apparatus which do not employ afriction-fusion process could be used. For example, a central plungermember and a peripheral member similar to the plunger 92 and peripheralmember 80, respectively, may be provided in a non-rotating ornon-orbiting system. Suitable means, such as an electric-resistanceheated blade could be employed to initially contact the facing surfacesof the overlapping strap lengths to melt a portion of the thickness ofeach strap length. Then, the heated blade could be withdrawn, and thestrap portions could be pressed together between the anvil on one sideand the central plunger and peripheral member on the other side.Subsequently, the peripheral member could be retracted (or the anvilelevated) to leave only the central plunger engaged so as to continue toapply pressure to the central region of the strap weld area. Finally,the anvil and central plunger would be separated sufficiently toterminate the application of pressure and permit access to the formedstrap weld.

Such a process could be employed with a strap material, such aspolyester, which has a tendency to release vapor to form cavities orbubbles in the weld. However, such a process could also be employed withother types of strap material which have little or no tendency toproduce encapsulated cavities or bubbles in a strap joint.

It will be appreciated that the novel apparatus of the present inventioncan be embodied in a design that can be relatively easily assembled andthat has operational advantages.

Further, with certain types of strap, a strap weld with improvedstrength characteristics can be provided. Also, the process of thepresent invention facilitates the production of high quality welds in aconsistent and repeatable manner.

It will be readily apparent from the foregoing detailed description ofthe invention and from the illustrations thereof that numerousvariations and modifications may be effected without departing from thetrue spirit and scope of the novel concepts or principles of thisinvention. It is therefore to be understood that within the scope of theappended claims, the present invention may be practiced otherwise thanas specifically described herein.

What is claimed is:
 1. A welded joint defined between two lengths of thermoplastic strap, comprising:said two lengths of thermoplastic strap being disposed in a face-to-face overlapping relationship with respect to each other at a location at which said welded joint is to be defined; at least part of the thickness of each one of said two strap lengths being resolidified from a fused state during which said two lengths of said strap disposed at said location at which said welded joint is to be defined were subjected to above-ambient pressure; said resolidified thickness part of one of said two strap lengths being merged with the resolidified thickness part of the other one of said two strap lengths so as to define a continuous resolidified region; and said resolidified region including a peripheral region, and an adjacent central region having a substantially disc-like configuration, wherein said above-ambient pressure was maintained, during said fused state, for a longer period of time within said central region than within said peripheral region.
 2. The joint in accordance with claim 1 in which said strap is polyester and said peripheral resolidified region includes a plurality of closed cavities.
 3. The joint as set forth in claim 2, wherein:said central region is substantially free of said cavities.
 4. The joint as set forth in claim 1, wherein:said strap is fabricated from polyethylene terephthalate.
 5. The joint as set forth in claim 1, wherein:the thickness of each one of said lengths of strap is approximately 0.5 mm; the width of each one of said lengths of strap is approximately 11 mm; the length of said welded joint, along the length of said strap, is approximately 17 mm; and the thickness of said welded joint comprising said resolidified region is approximately 0.1 mm.
 6. A welded joint defined between two lengths of thermoplastic strap, comprising:said two lengths of thermoplastic strap being disposed in a face-to-face overlapping relationship with respect to each other at a location at which said welded joint is to be defined; at least part of the thickness of each one of said two strap lengths being resolidified from a fused state; said resolidified thickness part of one of said two strap lengths being merged with the resolidified thickness part of the other one of said two strap lengths so as to define a continuous resolidified region; and said overlapping lengths of said strap disposed in said fused state and at said location at which said welded joint is to be defined comprising a central weld region, having a substantially circular configuration, and an adjacent peripheral weld region, which substantially annularly surrounds said central weld region, wherein said central and peripheral weld regions are subjected to above-ambient pressure, and wherein further, said above-ambient pressure is firstly released within said peripheral weld region and subsequently released within said central weld region.
 7. The joint as set forth in claim 6, wherein:said strap is fabricated from polyester.
 8. The joint as set forth in claim 7, wherein:said strap comprises polyethylene terephthalate.
 9. The joint as set forth in claim 6, wherein:said peripheral weld region comprises a plurality of closed cavities; and said central weld region is substantially free of cavities.
 10. The joint as set forth in claim 6, wherein:the thickness of each one of said lengths of strap is approximately 0.5 mm; the width of each one of said lengths of strap is approximately 11 mm; the length of said welded joint, along the length of said strap, is approximately 17 mm; and the thickness of said welded joint comprising said resolidified region is approximately 0.1 mm.
 11. A welded joint defined between two lengths of thermoplastic strap, comprising:two lengths of thermoplastic strap disposed in a face-to-face, overlapping relationship with respect to each other at a location at which a welded joint is to be defined; at least part of the thickness of each one of said two strap lengths being resolidified from a fused state wherein said two lengths of said strap disposed at said location at which said welded joint is to be defined are subjected to above-ambient pressure; said resolidified thickness part of one of said two strap lengths being merged with said resolidified thickness part of the other one of said two strap lengths so as to define a continuous resolidified weld region which defines said welded joint; and said continuous resolidified weld region which defines said welded joint comprises a central weld region at which said above-ambient pressure was maintained for a first predetermined period of time, and a peripheral weld region which substantially surrounds said central weld region and at which said above-ambient pressure was maintained for a second predetermined period of time which was shorter than said first predetermined period of time during which said above-ambient pressure was still maintained upon said central weld region.
 12. A welded joint as set forth in claim 11, wherein:said central weld region is substantially free of cavities.
 13. A welded joint as set forth in claim 11, wherein:said peripheral weld region comprises a plurality of closed cavities.
 14. A welded joint as set forth in claim 11, wherein:said strap is fabricated from polyester.
 15. A welded joint as set forth in claim 11, wherein:said strap is fabricated from polyethylene terephthalate.
 16. A welded joint as set forth in claim 11 wherein:the thickness of each one of said lengths of strap is approximately 0.5 mm; the width of each one of said lengths of strap is approximately 11 mm; the length of said welded joint, along the length of said strap, is approximately 17 mm; and the thickness of said welded joint comprising said resolidified region is approximately 0.1 mm.
 17. A welded joint defined between two lengths of thermoplastic strap, comprising:said two lengths of thermoplastic strap being disposed in a face-to-face overlapping relationship with respect to each other at a location at which said welded joint is to be defined; at least part of the thickness of each one of said two strap lengths being resolidified from a fused state during which said two lengths of said strap disposed at said location at which said welded joint is to be defined were subjected to above-ambient pressure; said resolidified thickness part of one of said two strap lengths being merged with the resolidified thickness part of the other one of said two strap lengths so as to define a continuous resolidified region; and said resolidified region including a peripheral region and an adjacent central region, wherein said peripheral region substantially surrounds said central region, and wherein further, said above-ambient pressure was maintained, during said fused state, for a longer period of time within said central region than within said peripheral region.
 18. The joint as set forth in claim 17, wherein:said central region is substantially free of cavities; and said peripheral region comprises a plurality of closed cavities.
 19. The joint as set forth in claim 17, wherein:said strap is fabricated from polyester.
 20. The joint as set forth in claim 17, wherein:said strap is fabricated from polyethylene terephthalate. 